Reactor and in situ FTIR studies of Pt/BaO/Al2O3 and Pd/BaO/Al2O3 NOx storage and reduction (NSR) catalysts

The reduction of NO under cyclic “lean”/“rich” conditions was examined over two model 1 wt.% Pt/20 wt.% BaO/Al₂O₃ and 1 wt.% Pd/20 wt.% BaO/Al₂O₃ NO_x storage reduction (NSR) catalysts. At temperatures between 250 and 350 °C, the Pd/BaO/Al₂O₃ catalyst exhibits higher overall NO_x reduction activity. Limited amounts of N₂O were formed over both catalysts. Identical cyclic studies conducted with non-BaO-containing 1 wt.% Pt/Al₂O₃ and Pd/Al₂O₃ catalysts demonstrate that under these conditions Pd exhibits a higher activity for the oxidation of both propylene and NO. Furthermore, in situ FTIR studies conducted under identical conditions suggest the formation of higher amounts of surface nitrite species on Pd/BaO/Al₂O₃. The IR results indicate that this species is substantially more active towards reaction with propylene. Moreover, its formation and reduction appear to represent the main pathway for the storage and reduction of NO under the conditions examined. Consequently, the higher activity of Pd can be attributed to its higher oxidation activity, leading both to a higher storage capacity (i.e., higher concentration of surface nitrites under “lean” conditions) and a higher reduction activity (i.e., higher concentration of partially oxidized active propylene species under “rich” conditions). The performance of Pt and Pd is nearly identical at temperatures above 375 °C.     

Reactivity of Mg-Ba catalyst in NOx storage/reduction

A new catalyst for NO_x storage/reduction was prepared to improve the activity of Ba-Pt/γ-Al₂O₃ by replacing Ba with a mixture of Ba and Mg. The catalyst was prepared by impregnating 1 wt.% Pt and then the alkaline-earth metals (Mg, Ba) on commercial γ-Al₂O₃. The tests have been carried out in a wide temperature range (ca. 200–400 °C) in order to understand the role of the mixture of alkaline-earth metals as a function of temperature. The behaviour of the two catalysts was different and indicated a synergetic effect between Mg and Ba.     

Pt添加方式对Mn基氧化物催化剂低温NSR活性的影响

分别采用Pt直接浸渍在Mn2O3上、Pt/Al2O3与Mn2O3分层耦合以及机械混合3种不同制备方法合成出Pt质量分数为1%的Mn基催化剂,并考察了其NOx储存性能以及NOx储存-还原循环活性。结果表明,机械混合的样品具有最大的NOx储存量和最高的消除效率。与Mn2O3样品相比较,机械混合的样品在150℃时的NOx储存量提高了2倍以上,高达368.1μmol·g-1,同时NOx的消除效率从0提高到48.1%。这是由于机械混合样品中Pt能够促使O2的解离,提高NOx储存效率;另外,机械混合样品中Pt与Mn的协同作用显著,从而提高了催化剂的NSR活性。     

Mean field modelling of NOx storage on Pt/BaO/Al2O3

A mean field model, for storage and desorption of NO_x in a Pt/BaO/Al₂O₃ catalyst is developed using data from flow reactor experiments. This relatively complex system is divided into five smaller sub-systems and the model is divided into the following steps: (i) NO oxidation on Pt/Al₂O₃; (ii) NO oxidation on Pt/BaO/Al₂O₃; (iii) NO_x storage on BaO/Al₂O₃; (iv) NO_x storage on Pt/BaO/Al₂O₃ with thermal regeneration and (v) NO_x storage on Pt/BaO/Al₂O₃ with regeneration using C₃H₆. In this paper, we focus on the last sub-system. The kinetic model for NO_x storage on Pt/BaO/Al₂O₃ was constructed with kinetic parameters obtained from the NO oxidation model together with a NO_x storage model on BaO/Al₂O₃. This model was not sufficient to describe the NO_x storage experiments for the Pt/BaO/Al₂O₃, because the NO_x desorption in TPD experiments was larger for Pt/BaO/Al₂O₃, compared to BaO/Al₂O₃. The model was therefore modified by adding a reversible spill-over step. Further, the model was validated with additional experiments, which showed that NO significantly promoted desorption of NO_x from Pt/BaO/Al₂O₃. To this NO_x storage model, additional steps were added to describe the reduction by hydrocarbon in experiments with NO₂ and C₃H₆. The main reactions for continuous reduction of NO_x occurs on Pt by reactions between hydrocarbon species and NO in the model. The model is also able to describe the reduction phase, the storage and NO breakthrough peaks, observed in experiments.     

Planar Models for Alumina-Based Catalysts

Planar transition aluminas are gaining attention as models for alumina-based catalysts [I] because of their attractiveness for study by modern surface analytical techniques [2], electron optical methods [3], and reflection spectroscopy [4–7]. In this context “planar” means a thin (<10–4 cm) flat oxide layer of uniform thickness. Conventional high surface area aluminas which are used commercially for adsorbents, catalysts, and catalyst supports usually require grinding and /or pressing into disks for characterization by these approaches. This can expose unwanted inner structures or expose the samples to potential contamination by the grinding and pressing tools or uncontrolled atmospheres. In addition, difficulties arise in attempting to study the various stages of alumina development involved with commercial methods since these entail sols, gels, powdered hydroxides, and other inconvenient structures.     

Overview of the Fundamental Reactions and Degradation Mechanisms of NOx Storage/Reduction Catalysts

Over the last several years, nitrogen oxide(s) (NOx) storage/reduction (NSR) catalysts, also referred to as NOx adsorbers or lean NOx traps, have been developed as an aftertreatment technology to reduce NOx emissions from lean-burn power sources. NSR operation is cyclic: during the lean part of the cycle, NOx are trapped on the catalyst; intermittent rich excursions are used to reduce the NOx to N₂ and restore the original catalyst surface; and lean operation then resumes. This review will describe the work carried out in characterizing, developing, and understanding this catalyst technology for application in mobile exhaust-gas aftertreatment. The discussion will first encompass the reaction process fundamentals, which include five general steps involved in NOx reduction to N₂ on NSR catalysts; NO oxidation, NO₂ and NO sorption leading to nitrite and nitrate species, reductant evolution, NOx release, and finally NOx reduction to N₂. Major unresolved issues and questions are listed at the end of each of the reaction process fundamental sections. Degradation mechanisms and their effects on NSR catalyst performance are also described in relation to these generalized reactions. Since at this stage it does not appear possible to arrive at a complete and consistent mechanistic model describing the broad, existing experimental phenomenology for these processes, this review is primarily focused on summarizing and evaluating literature data and reconciling the many differences presented.     

Knowledge and Know-How in Emission Control for Mobile Applications

Concerns about emissions of carbon dioxide have created a need to develop more fuel-efficient vehicles. Diesel engines are generally more efficient than gasoline engines but improvements in the latter can be achieved by operating under lean-burn conditions. With both diesel and lean-burn gasoline engines, the nitrogen oxides are emissted under oxidising conditions. It is scientifically very challenging to reduce nitrogen oxides under oxidising conditions. After a short survey of conventional three-way catalysts, and the associated fundamental aspects of NO_x reduction under stoichoimetric conditions, this review focuses on the knowledge and know-how that has been developed for lean engine emission control. Early research on hydrocarbon selective catalytic reduction on zeolite, oxide, and metal-based systems is examined, and some of the key mechanistic models are described. Since none of these systems are of sufficient activity and stability to satisfy current legislation attention has turned to NO_x storage and reduction systems. The basic principles of these are described, and the present state of knowledge regarding the mechanisms of storage and regeneration are discussed. The many apparent discrepancies are highlighted and an attempt is made to rationalise the current state of knowledge by taking into account the varying experimental conditions reported nit he literature. For diesel engines, NO_x storage and reduction is not an ideal solution and so the final section of this review is concerned with silver-based catalysts and especially with the dramatic effect of small amounts of hydrogen on the efficiency of these catalysts for hydrocarbons selective catalytic reduction.     

Insight into the key aspects of the regeneration process in the NOx storage reduction (NSR) reaction probed using fast transient kinetics coupled with isotopically labelled NO over Pt and Rh-containing Ba/Al2O3 catalysts

For the first time, the coupling of fast transient kinetic switching and the use of an isotopically labelled reactant (¹⁵NO) has allowed detailed analysis of the evolution of all the products and reactants involved in the regeneration of a NO_x storage reduction (NSR) material. Using realistic regeneration times (ca. 1 s) for Pt, Rh and Pt/Rh-containing Ba/Al₂O₃ catalysts we have revealed an unexpected double peak in the evolution of nitrogen. The first peak occurred immediately on switching from lean to rich conditions, while the second peak started at the point at which the gases switched from rich to lean. The first evolution of nitrogen occurs as a result of the fast reaction between H₂ and/or CO and NO on reduced Rh and/or Pt sites. The second N₂ peak which occurs upon removal of the rich phase can be explained by reaction of stored ammonia with stored NO_x, gas phase NO_x or O₂. The ammonia can be formed either by hydrolysis of isocyanates or by direct reaction of NO and H₂.

The study highlights the importance of the relative rates of regeneration and storage in determining the overall performance of the catalysts. The performance of the monometallic 1.1%Rh/Ba/Al₂O₃ catalyst at 250 and 350 °C was found to be dependent on the rate of NO_x storage, since the rate of regeneration was sufficient to remove the NO_x stored in the lean phase. In contrast, for the monometallic 1.6%Pt/Ba/Al₂O₃ catalyst at 250 °C, the rate of regeneration was the determining factor with the result that the amount of NO_x stored on the catalyst deteriorated from cycle to cycle until the amount of NO_x stored in the lean phase matched the NO_x reduced in the rich phase. On the basis of the ratio of exposed metal surface atoms to total Ba content, the monometallic 1.6%Pt/Ba/Al₂O₃ catalyst outperformed the Rh-containing catalysts at 250 and 350 °C even when CO was used as a reductant.     

Scanning tunneling microscopy and spectroscopy of silver and platinum catalysts

Scanning tunneling microscopic studies of silver catalysts dispersed on highly oriented pyrolitic graphite (HOPG) with 2, 5 and 10 wt% metal loading prepared by wet impregnation and hydrogen reduction show spherical crystallites, with the 2 wt% catalysts having an average crystallite size of 2 nm and the 5 and 10 wt% catalysts with size distributions of 2–5 and 4–12 nm respectively. The Ag catalysts prepared by NaBH₄ reduction show a narrower size distribution. Pt/HOPG catalysts with 2 and 5 wt% metal loading prepared by wet-impregnation and hydrogen reduction show large (2–11 nm) raft-like crystallites; small crystallites ( 1 nm) could be obtained by NaBH₄ reduction. Tunneling spectroscopic measurements reveal the nonmetallic nature of crystallites on the surface of Ag(2 wt%)/HOPG as well as Pt/HOPG prepared by NaBH₄ reduction.     

Scanning Tunneling Microscopy (STM) at High Pressures. Adsorption and Catalytic Reaction Studies on Platinum and Rhodium Single Crystal Surfaces

In this paper, we review the experimental results that were obtained using our high pressure–high temperature STM system (HP/HT STM) for studies of gas adsorption in a broad pressure range. Measurements are carried out in equilibrium with the gas phase. We discovered ordered surface structures of adsorbates that do not exist at low pressures. It appears that small increases in coverage due to increased ambient pressures cause ordering due to repulsive adsorbate–adsorbate interactions. Adsorption isotherms, one of the oldest fields of surface thermodynamics, can be revisited using HP/HT STM to obtain molecular surface structures and surface phase diagrams as the gas pressure is altered.     

Performance features of Pt/BaO lean NOx trap with hydrogen as reductant

The performance of a model Pt/BaO/Al₂O₃ monolith catalyst was studied using H₂ as the reductant. The dependence of product selectivities on operating parameters is reported, including the durations of regeneration and storage times, feed composition and temperature, and monolith temperature. The data are explained in terms of a phenomenological model factoring in the transport, kinetic, and spatio‐temporal effects. The Pt/BaO catalyst exhibits high cycle‐averaged NOx conversion above 100°C, generating a mixture of N₂ and byproducts NH₃ and N₂O. The cycle‐averaged NOx conversion exhibits a maximum at about 300°C corresponding to the NOx storage maximum. The N₂ selectivity exhibits a maximum at a somewhat higher temperature, at which point the NH₃ selectivity exhibits a minimum. This trend conveys the intermediate role of NH₃ in reacting with stored NOx. Both N₂ and N₂O are also formed during the storage steps from the oxidation of NH_x species produced during the regeneration. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Abatement of diesel-exhaust pollutants: NOx storage and soot combustion on K/La2O3 catalysts

Potassium-loaded lanthana is a promising catalyst to be used for the simultaneous abatement of soot and NO_x, which are the main diesel-exhaust pollutants. With potassium loadings between 4.5 and 10 wt.% and calcination temperatures between 400 and 700 °C, this catalyst mixed with soot gave maximum combustion rates between 350 and 400 °C in TPO experiments, showing a good hydrothermal stability. There was no difference in activity when it was either mixed by grinding in an agate mortar or mixed by shaking in a sample bottle (tight and loose conditions, respectively). Moreover, when the K-loaded La₂O₃ is used as washcoat for a cordierite monolith, there were found no significant differences in the catalytic behaviour of the system, which implies its potentiality for practical purposes.

The influence of poisons as water and SO₂ was investigated. While water does not affect the soot combustion activity, SO₂ slightly shift the TPO peak to higher temperature. Surface basicity, which is a key factor, was analysed by measuring the interactions of the catalytic surface with CO₂ using the high frequency CO₂ pulses technique, which proved to be very sensitive, detecting minor changes by modifications in the dynamics of the CO₂ adsorption–desorption process. Water diminishes the interaction with CO₂, probably as a consequence of an adsorption competition. The SO₂ treated catalyst is equilibrated with the CO₂ atmosphere more rapidly if compared with the untreated one, also showing a lower interaction. The lower the interaction with the CO₂, the lower the activity.

Differential scanning calorimetric (DSC) results indicate that the soot combustion reaction coexists with the thermal decomposition of hydroxide and carbonate species, occurring in the same temperature range (350–460 °C). The presence of potassium increases surface basicity shifting the endothermic decomposition signal to higher temperatures.

We also found that NO₂ strongly interacts with both La₂O₃ and K/La₂O₃ solids, probably through the formation of monodentate nitrate species which are stable under He atmosphere until 490 °C. These nitrate species further react with the solid to form bulk nitrate compounds. The addition of Cobalt decreases the nitrates stability and catalyses the NO_x to N₂ reduction under a reducing atmosphere, which is a necessary step for a working NO_x catalytic trap. Preliminary studies performed in this work demonstrated the feasibility of using these catalysts to simultaneously remove NO_x and soot particles from diesel exhausts. The nitrate formation is still observed during the catalytic combustion of soot in the presence of NO_x, making our K/La₂O₃ a very interesting system for practical applications in simultaneous soot combustion and NO_x storage in diesel exhausts.     

Selectivity-determining role of C3H8/NO ratio in the reduction of nitric oxide by propane in presence of oxygen over ZSM5 zeolites

The reaction of (NO + C₃H₈ + O₂) can result in selective formation of NO₂ over H-ZSM5, Cu,H-ZSM5, Ag,H-ZSM5, and Li,H-ZSM5 catalysts when the concentrations of NO and O₂ are 0.1 and 9%, SV > 60,000 h⁻¹ (typical for automotive exhausts), and C₃H₈/NO > 1. Despite stoichiometric excess of reductant hydrocarbon below this limit, the in situ formed NO₂ does not react with C₃H₈, thus conversion of NO to N₂ is negligible. NO can be reduced by C₃H₈ selectively to N₂ only when C₃H₈/NO ≧ 1. Contrary to many suggestions the reaction temperature, concentration of oxygen, space velocity, and type of exchange ions have minor influence on the selectivity for N₂. These parameters affect the rates of reactions (NO + ₂), (C₃H₈ + NO_x) and (C₃H₈ + O₂), therefore they also affect the production of N₂ in the HC-SCR process, but only when the ratio of C₃H₈/NO permits. The metal-exchanged zeolites were prepared in situ by solid-state ion exchange from H-ZSM5. Despite the low degree of copper exchange (63%), Cu,H-ZSM5 produces substantially more N₂ than H-ZSM5, Ag,H-ZSM5, or Li,H-ZSM5. However, the selectivity for N₂ is lowest over Cu,H-ZSM5, which also produces considerable NO₂ in the reaction of (NO + C₃H₈ + O₂) even at C₃H₈/NO ≧ 1. Contrary to prior findings, the catalytic activity of Cu,H-ZSM5 for the oxidation of NO by O₂ to NO₂ in absence of hydrocarbon was comparable to that of H-ZSM5 at high space velocities (2.3 l g⁻¹ min⁻¹). By replacing 30 and 40% of the protons of H-ZSM5 by Ag⁺ and Li⁺ ions in Ag,H-ZSM5 and Li,H-ZSM5, respectively, the catalytic activity for this reaction becomes negligible at temperatures ≧100°C. Some mechanistic consequences of these experimental observations are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Use of experimental design in development of a catalyst system

NO_x storage and reduction experiments have been performed with stationary operation of a heavy-duty diesel engine rig. An optimization of the NO_x reduction performance has been done using experimental design. The adjustable parameters in this study were cycle time, injection time, injection rate and bypass time (period of reduced flow through catalysts). NO_x was reduced by 50–60% (3.3–4.1 g/kWh) with a fuel penalty below 5%. It was shown that experimental design was efficient for optimizing the NO_x reduction and this systematic approach enabled important conclusions to be drawn about the system performance.     

Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts

This study provides insight into the effect of Pt dispersion on the overall rate and product distribution during NOx storage and reduction. The storage and reduction performance of Pt/BaO/A₂O₃ monoliths with varied Pt dispersion (3%, 8%, and 50%) and fixed Pt (2.48 wt.%) and BaO (13.0 wt.%) loadings is reported. At low temperature (<200 °C), the differences in storage and reduction activity were the largest between the three catalysts. The amount of NOx stored increased with increased dispersion, as did the amount of stored NOx that was reduced. These trends are attributed to larger Pt surface area and Pt–BaO interfacial perimeter, the latter of which enhances the spillover of surface species between the precious metal and storage components. At high temperature (370 °C), the stored NOx was almost completely regenerated for the three catalysts. However, the regeneration of the 3% dispersion catalyst was much slower, suggesting a rate limitation involving the reverse spillover of stored NOx to Pt and/or of adsorbed hydrogen from Pt to BaO. The results indicate that the catalyst dispersion and operating conditions may be tuned to achieve the desired ammonia selectivity. For the aerobic regeneration feed, the most (net) NH₃ was generated by the 50% dispersion catalyst at the lowest temperature (125 °C), by the 3% dispersion catalyst at the highest temperature (340 °C), and by the 8% dispersion catalyst at the intermediate temperatures (170–290 °C). Similar trends were observed for the net production of NH₃ with an anaerobic regeneration feed. A phenomenological picture is proposed that describes the effects of Pt dispersion consistent with the established spatio-temporal behavior of the lean NOx trap.     

Transient kinetic study of the SCR-DeNOx reaction

The unsteady-state kinetics of the selective catalytic reduction (SCR) of NO with NH₃ is studied over V₂O₅–WO₃/TiO₂ model catalysts by means of the transient response method. NH₃ strongly adsorbs onto the catalyst surface whereas NO does not adsorb appreciably. A dynamic mathematical model based on a Temkin-type desorption process for NH₃ and a SCR reaction rate with a complex dependence on the ammonia surface coverage is well suited to represent the data.     

三效催化剂中促进剂氧化铈的作用研究进展

综述了CeO2在三效催化剂中的应用研究现状。包括：在贫燃和富燃条件下起储存和释放氧的作用；促进贵金属的分散和增加Al2O3载体的稳定性；促进水煤气转化和水蒸气重整反应；在金属一载体界面上增加催化反应活性位并促使CO在晶格氧上有效氧化以及催化去除SOx气体。     

稀燃汽车尾气中氮氧化物的催化消除技术

稀薄燃烧(简称稀燃)技术能够使燃料在发动机内充分燃烧,既提高了燃油的经济性,同时又减少了温室气体CO2的排放,因而是一项节能减排的重要技术.但在稀燃条件下氧气大量过剩,加剧了三效催化剂对还原剂的催化氧化,降低了还原剂对NOx催化还原的效率.目前,国际上对稀燃气氛下NOx的消除主要采用NO直接分解、选择性催化还原(SCR...     

Multifunctional catalysts for de-NOx processes: The case of H3PW12O40·6H2O-metal supported on mixed oxides

The role of a multifunctional catalyst for de-NO_x process has been investigated. The NO_x storage capacity of H₃PW₁₂O₄₀·6H₂O (HPW) was improved by the presence of a noble metal (Pt, Rh or Pd). Both HPW and noble metal were deposited on a specific support (based on Zr–Ce or Zr–Ti). The presence of noble metal in several oxidation states, as evidenced by TPR and IR, involves the possibility of forming different catalytic sites: (i) M⁰ (zero-valent metal) and perhaps (ii) (metal–H)^δ+ from specific interactions between noble metal and the HPW proton. Supports were also able to adsorb and activate NO_x and to generate cationic catalytic sites (M^x+). These cationic sites seem to be the clue for their important activity toward NO_x reduction. This catalyst presents an outstanding resistance to SO₂ poisoning which can be related to NO and NO₂ absorption mechanism in HPW. The use of alternating short cycles of lean/rich mixtures allows us optimising the performance of this catalytic system in terms of both NO_x reduction capacity and NO_x storage efficiency: up to 48 and 84%, respectively (with a 2% CO + 1% H₂ mixture for reducing). Experimental results sustain two hypotheses: first, HPW-metal-support catalyst includes several (independent) catalytic functions required for a de-NO_x process to occur and second, the formation of oxygenate active species must be indispensable for NO_x reduction into nitrogen.     

Revisiting the steady states of NO/O2/C3H6 on monolithic Pt/BaO/Al2O3 using bifurcation analysis

While NOx storage and reduction is periodically operated, steady‐state studies have been widely carried out to investigate the involved reaction mechanisms and effects of operating parameters. Due to the complex reaction chemistry and its coupling with transport phenomena, multiplicity may exist. A steady‐state monolith reactor model accounting for microkinetics and reaction heat effects was proposed in this study to avoid the evaluation of enthalpies of microreaction steps. Three simplified versions of the monolith model were developed based on various assumptions of the axial gradients. Steady‐state behaviors of NO/O₂/C₃H₆ system were investigated. A predictor‐corrector (PC) continuation method that does not require explicit evaluation of Jacobian Matrix was developed to solve the nonlinear system with a variable parameter of feed temperature. Model predictions were compared with experimental results. © 2013 American Institute of Chemical Engineers AIChE J 60: 623–634, 2014